there are many reasons as everyone has said already but i read that the Western empire collapsed because much of the military and government were moved to Constantinople. Also the use of legionaires declined aswell and the roman army began to center around the use of cavalry, after adrianople....The barbarians swelled and expanded into the heart of the empire..because of the Huns-Attila. Theodosius also allowed the goths cross the Danube and settle.....there are many other reasons. But i guess i have said enough